Wednesday, March 24, 2010

I am interested in the role of phenotypic plasticity in adaptation to different environments. Divergent selection can result in adaptive genetic divergence among populations. However, individuals can also adapt through a plastic response; i.e. the environment might have a direct impact on the phenotype without influencing genetic change. The relative contribution of each (genetic vs. plastic adaptation) should be related to the level of dispersal and gene flow among selective environments. Gene flow can constrain adaptive genetic divergence, and therefore plasticity might be favoured under high gene flow scenarios. If heritable variation for plasticity occurs in a meta-population, I predict that plasticity might evolve in response to gene flow. At the same time, plasticity might permit increased dispersal and gene flow among environments.

During my doctoral studies at McGill, I developed a conceptual framework for understanding the above pathways. I then tested some of the framework's predictions in an empirical system: an African cichlid fish from high-oxygen rivers and adjacent low-oxygen swamps. I found that morphological plasticity, in response to dissolved oxygen concentration, was high overall in this species, but that plasticity was higher at locations where dispersal between environments should also be higher. I infer that plasticity might have evolved in response to gene flow between oxygen environments, but more studies are needed to determine causality.

The conceptual framework is published in the following article. The empirical studies are currently in review.Crispo, E. (2008) Modifying effects of phenotypic plasticity on interactions among natural selection, adaptation and gene flow. Journal of Evolutionary Biology 21:1460-1469

Sunday, March 14, 2010

Evolution is obviously driven by ecological differences: think of the adaptive radiation of Darwin’s finches. Just as obviously, ecological processes are influenced by evolution: ecosystems depend on the oxygen produced following the evolution of photosynthesis. Less obvious is how these interactions play out on the short time scales most relevant to conservation and management. Do ecological changes (e.g., invasive species, climate change) drive appreciably evolutionary change over years or decades (i.e., “contemporary evolution”)? Does any such evolution influence ecological variables (population dynamics, community composition, ecosystem function) on similar time scale? These potential interactions between ecology and evolution, as shown in the figure, represent the growing field of eco-evolutionary dynamics.

Many studies have shown that ecological changes cause phenotypic changes in natural populations (eco-to-evo). Examples include species introduced to new environments, native species responding to introduced species, populations exposed to harvesting or pollution, and populations facing climate change. Existing work has shown that the phenotypic changes can be substantial, particularly when humans are involved. What isn’t known generally is just how much of this phenotypic change is the result of evolutionary change versus phenotypic plasticity. Even less is known about how these contemporary phenotypic changes then influence ecological variables on similar time frames (evo-to-eco) – but some nice examples can be provided.

Populations: Phenotypic changes from one year to the next clearly influence population size in ungulates. The genetic contribution to this phenotypic change is not known, whereas a study of butterflies has documented effects of genetic change on population sizes. What remains to be determined is just how common these effects are, and how important they are relative to traditional “ecological” effects (e.g., rainfall or temperature). In addition, it isn’t clear under which conditions these population dynamical effects of evolution can actually save natural populations from extinction (i.e., evolutionary rescue).

Communities: Genetic and phenotypic differences between individual plants have been shown to have noteworthy effects on arthropod communities. Similarly, genetically-based phenotypic differences between fish populations have strong influences on aquatic macro-invertebrate communities. What remains to be determined is, again, how common these effects are and, also, how year-to-year changes in these genes and traits (as opposed to the currently-studied static differences) influence those communities.

Ecosystems: In the same plants and fish studied for community effects (above), genetic and phenotypic differences have been shown to influence ecosystem variables such as decomposition rates, dissolved organic material, light attenuation, and primary productivity. Since the study systems are the same as above, what remains to be discovered is also the same. It will also be interesting to know how often these ecosystem effects of evolution fall into the category of “ecosystem services” that have become so integral to conservation efforts.

The above listing highlights a few specific examples of how evolutionary change might influence ecological variables on short time scales. In addition to the specific uncertainties listed above, some additional general ones come to mind. How often do evolutionary effects on communities and ecosystems flow through the effects of evolution on population dynamics (indirect effects – red to black arrows in the figure) versus changes in the traits themselves (direct effects – red arrows only)? Do the effects of evolutionary change on ecological processes decrease from population to community to ecosystem variables? How often do true feedbacks occur – that is an ecological process drives evolution (green arrows) that then alters that same ecological process (red arrows) and so on? Eco-evolutionary dynamics is an area ripe for future work.

A special journal issue on eco-evolutionary dynamics: http://rstb.royalsocietypublishing.org/content/364/1523

Friday, March 5, 2010

On February first Xavier submitted his thesis "The driving factors of ecological speciation and their interactions" in which he looked at the effects of gene flow on local adaptation, and reciprocally, the effects of local adaptation on gene flow.